This invention relates generally to the field of spectroscopy, and, more particularly, to an improved technique of double-beaming in Fourier spectroscopy.
Spectroscopy is concerned with the production, measurement and interpretation of electromagnetic spectra arising from either emission or absorption of radiant energy by various substances. Emission spectra are produced when radiant energy from matter, excited by various forms of energy is passed through a slit and subsequently separated into its various components or wavelengths either by refraction in a transparent prism or by diffraction from a ruled grating or in a crystalline solid. The production of absorption spectra involves use of similar devices except that the excitation source is replaced by a suitable radiant energy source. Spectroscopic measurements of wavelengths and intensities of radiative energy are made using instruments called spectroscopes, spectrographs, spectrometers, or spectrophotometers.
Interpretation of the spectra provides information concerning atomic and molecular energy levels, electronic configurations of atoms and ions, molecular geometries, and chemical bonds. Empirical correlations of the spectral characteristics with chemical and physical properties of matter provide a basis for qualitative and quantitative chemical analysis.
The form of spectroscopy which has been found to be the most superior is that of Fourier spectroscopy. There are many reasons for this superiority, but the two most often referred to are the multiplex advantage and the aperture advantage. In addition, absolute wave number accuracy is guaranteed by the known wavelength used for carriage control, the physical apparatus is simple and both stray light and overlapping spectral orders are eliminated.
Unfortunately there are still a number of problems associated with Fourier spectroscopy. One of these is the dynamic range problem and another is the peculiar form of the raw data. It is not the spectrum which is measured, but rather a quantity called the interferogram, which is related to the spectrum by a Fourier transformation.
An attempt at the solution of the dynamic range problem is a double-beaming differencing interferometer. Such a system operates on the principle of looking alternatively at a reference and sample beam. However, all previous techniques of double-beaming consist of sequential measurements of short or large delays (between measurements) depending on the phenomenon being studied, and require either two detectors or a chopper which sends the two beams alternately on one detector. These drawbacks in double-beaming Fourier spectroscopy must be overcome for this technique to be readily accepted in the field of spectroscopy.